Vinylidene fluoride polymer based coatings are well known. They are used extensively for protection or decoration or both of a variety of substrates. These coatings are also known for their exceptional durability with resistance to the effect of weathering and adverse chemical environments. A particularly useful category of vinylidene fluoride polymer coatings is the well-known architectural paint finish for building panels and other articles of sheet metal and metal extrusions. Various systems have been devised for laying down a film-forming composition of normally solid vinylidene fluoride polymers to give the substrates the desired long term protection against the effect of chemicals and weather. U.S. Pat. No. 3,324,069 to Koblitz et al. (1967) disclosed a composition comprising a vinylidene fluoride polymer, a latent solvent for said vinylidene fluoride polymer, and an acrylate polymer dissolved in said latent solvent. U.S. Pat. No. 3,340,222 to Fang (1967) disclosed a liquid film-forming composition comprising a volatile organic liquid, a vinylidene fluoride polymer, and adsorbed on the pigment an acrylate polymer. The pigmented organosol dispersions or paint finishes are applied by conventional means to the metallic substrate. After application, the coated substrate is baked at sufficiently high temperature and time to drive off the solvent and fuse the vinylidene fluoride polymer film to the substrate. U.S. Pat. No. 4,400,487 to Stoneberg et al. (1983) disclosed textured coatings comprising a vinylidene fluoride polymer, an acrylic resin and a particulate fluoropolymer. Stoneberg mentioned that the acrylic resin can be also in latex form and is cited for reference. The increasing emphasis on environmental protection as defined by the Clean Air Act makes organic solvent containing film-forming compositions no longer desirable. Accordingly, there is a growing need for environmentally friendly paint finishes that are essentially free of organic solvents.
U.S. Pat. No. 4,022,737 to Sekamakas et al. (1977) disclosed stable aqueous dispersions of a vinylidene fluoride polymer powder and an acrylate resin latex. U.S. Pat. No. 4,128,519 to Bartoszek et al. (1978) disclosed water-based vinylidene fluoride polymer coatings that contain an emulsified liquid epoxy resin. U.S. Pat. No. 4,141,873 to Dohany (1979) disclosed aqueous dispersions of a vinylidene fluoride polymer and admixed therewith an aqueous dispersion of an acrylate polymer and a water-soluble organic solvent acting as film-coalescing agent. U.S. Pat. No. 4,309,928 to Carson et al. (1982) disclosed methods for producing stable aqueous dispersions of vinylidene fluoride polymer powder. U.S. Pat. No. 4,383,075 to Abel (1983) disclosed an aqueous thermosetting coating composition that comprise a reactive group containing high T.sub.g acrylic resin latex and a vinylidene fluoride polymer.
Vinylidene fluoride polymer based powder coatings are suitable alternative to organosol or aqueous dispersions. Processes for the preparation of film-forming vinylidene fluoride polymer based powder coatings are disclosed in U.S. Pat. Nos. 4,770,939 to Sietses at al. (1988), 5,030,394 to Sietses et al. (1991), 5,177,150 to Polek (1993), 5,229,460 to Yousuf et al. (1993), 5,346,727 to Simkin (1994), and 5,405,912 to Simkin (1995). The processes of the prior art require that the ingredients of the compositions comprising a vinylidene fluoride polymer, an acrylite polymer, and at least one pigment be first melt compounded from about 190.degree. C. to 240.degree. C. using an extruder, roller mill, or any other appropriate heated mixing device. Thereafter the melt mixed compound is pelletized and milled at cryogenic temperature from about -50.degree. C. to about -150.degree. C. using liquid nitrogen as coolant and sieved to obtain a powder having specified particle size.
These processing steps suffer from several disadvantages. They require significant amount of energy to extrude and pelletize the polymer compound, and to cryogenically grind the pellets to obtain the powder. The cryogenic grinding step also consumes a large amount of liquid nitrogen that is used as coolant. After cryogenic grinding, if desired or required, the powder is classified and larger particles are usually recycled to the mill for more grinding as described in U.S. Pat. No. 5,030,394. Regrinding of larger particles adds to the consumption of liquid nitrogen and energy input. A change from one color to another requires purging of the extruder of the compound having the first color by introducing a purge compound or the compound of the second color. This process inevitably produces an intermediate product of an undesired color that must be discarded as scrap, thereby resulting in waste of material and time.
Accordingly, there is a need for a method that avoids extrusion compounding of the blend and cryogenic grinding of pellets to powder. In the U.S. Pat. No. 5,739,202 (1998) Pecsok disclosed a novel process for the manufacture of pigmented film-forming vinylidene fluoride polymer powders suitable for powder coating applications. This process avoids the use of extrusion or melt compounding and cryogenic grinding of the pellets. However, the process uses a volatile and flammable organic solvent during the preparation of the powder coating compositions. Accordingly, there is a need for an improved process that does avoid the use of any volatile organic solvent during the preparation of powder coatings and avoids melt compounding of blends and cryogenic grinding of pellets to powder.